There are a variety of communication system applications that require extremely low distortion, highly linear signal driver components. As a non-limiting example, consider the case of a relatively long (on the order of 20-25 kft) two-wire, twisted subscriber loop pair serving a `remote` terminal for conducting full duplex communications. In order to prevent mutual interference between the transmit and received signals, echo-canceling circuitry is installed at the respective ends of the loop. However, the performance of these components, and therefore the effective service distance of the loop, is limited by the amount of distortion introduced into the signals being transported over the two-wire pair, and the ability of the echo-canceling circuitry to precisely excise the unwanted echo of the transmitted signal and still realize a useable signal that is received from the far end equipment.
To this end, the line driver components should be extremely linear and introduce minimum distortion, with the output impedance of the line driver being defined so as to match the characteristic impedance of the line. It is especially difficult to maintain this linearity when power is restricted, as in the case where the two-wire pair is also used for powering remote customer premises equipment (e.g., DSL, ISDN equipment) from the central office.
Recent advances in loop data transmission equipment components have made it possible to reduce the power supply requirements for local digital subscriber loop circuits from supply rails on the order of .+-.15 volts (or a thirty volt power rail differential), to .+-.8 volts power supply rails (a sixteen volt power supply differential). Driving the loop with components that operate with relatively small absolute signal voltage swings (e.g., five volts peak-to-peak) relative to the power supply differential improves linearity. However, it still leaves a considerable voltage difference (e.g., eleven volts for a .+-.8 volts supply) between the maximum signal excursion and the supply rail differential, resulting in a substantial standby power overhead loss (the product of the standby current times the voltage difference).
To reduce this standby power loss, the power overhead can be reduced to make the supply rail differential be as close as possible to the differential signal swing. However, this power reduction objective must be balanced against the fact such an overhead reduction inherently increases the amount of distortion in the line driver amplifiers.